The present invention relates to improvements in signal processing systems and more particularly to improved techniques for eliminating interference introduced into the side-lobes of an antenna from multiple interference sources.
Signal processing equipment in general is designed with a goal of receiving only particular information for evaluation. However, as is often the case, desired information is not isolated by itself but may be found in the presence of unwanted signals. Antenna systems in particular have characteristics that include a main lobe for receiving desired information and a plurality of side-lobes at various angles relative to the main lobe. Due to the nature of an antenna, information received in a side-lobe is indistinguishable from information received in the main lobe and thus renders the antenna highly susceptible to interference from unwanted signals or information. This problem is particularly acute in radar systems where the presence of side-lobes makes it possible for a single noise jammer to be effective against a radar from any angle of azimuth. The problem becomes even more acute when multiple interference or jamming sources are used against a radar and directed from a variety of directions simultaneously.
Side-lobe cancellation is a fundamental approach to eliminating interference in received signals and has been used relatively successfully to eliminate the interference introduced from a single jamming source. Generally, to provide successful cancellation, the side-lobe canceller uses a signal received by an auxiliary omnidirectional antenna to cancel the interference signal received in the side-lobe of the primary antenna. Such a system is taught by U.S. Pat. No. 3,202,990 to Paul W. Howells.
In the case of a plurality of interference or jamming sources, conventional side-lobe canceller systems have not been quite as successful in eliminating interference. As also shown by the patent to Howells, multiple auxiliary antennas distributed in space sample the electromagnetic interference in which the radar is attempting to operate and are combined with the interference in the main radar antenna in a manner designed to subtract the interfering signals. The system according to Howells, however, uses a parallel connection of canceller loops attempting to minimize the correlation between its auxiliary signal and the output of the main channel subtractor. Each canceller, however, can develop only one correlator output that must attenuate and phase shift all components of the auxiliary signal by the same amount. Due to the parallel connection, this can give rise to one canceller introducing signals that must be taken out by another when two or more uncorrelated interference signals are simultaneously received from different directions. This necessity for the cancellers to work against each other requires the canceller loops to have large dynamic ranges that introduce instabilities into the system and limit canceller effectiveness. In practice the instability has been reduced by limiting the gain in each loop, but the total effect is then to limit the amount of cancellation which in turn degrades the overall system performance.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide a device for increasing the cancellation and stability of interference suppression systems.